Blocking of the terminal hydroxyl groups and changing the molecular weight of polyorganosiloxanes having one hydroxyl group in each of the terminal units is well known in the art. For example, U.S. Pat. No. 4,203,913 to Burkhardt et al. describes a process in which .alpha.,.omega.-dihydroxypolydiorganosiloxanes are reacted with .alpha.,.omega.-bis-trimethylsiloxypolydiorganosiloxanes in the presence of phosphonitrile chlorides. Also, German Offenlegungsschrift No. 3,524,484 (J. Burkhardt, Wacker-Chemie GmbH), describes a method for replacing terminal hydroxyl groups of .alpha.,.omega.-dihydroxypolyorganosiloxanes by reacting the .alpha.,.omega.-dihydroxypolyorganosiloxanes with hexaorganodisilazane in the presence of phosphonitrile chlorides.
Therefore, is an object of the present invention to provide a process for blocking the terminal hydroxyl groups of polyorganosiloxanes (.alpha.,.omega.-dihydroxypolyorganosiloxanes) having hydroxyl groups in the terminal units. Another object of the present invention is to provide a process for blocking the terminal hydroxyl groups of the polyorganosiloxanes with a composition having a low volatility at the pressures and temperatures employed in the blocking reaction. Still another object of the present invention is to provide a process for blocking the hydroxyl groups and changing the molecular weight of .alpha.,.omega.-dihydroxypolyorganosiloxanes. A further object of the present invention is to provide a process for blocking the hydroxyl groups and changing the molecular weight of .alpha.,.omega.-dihydroxypolyorganosiloxanes which makes it posible to use less catalyst at a comparable space-time yield. A still further object of the present invention is to provide a process for blocking the hydroxyl groups and changing the molecular weight of .alpha.,.omega.-dihydroxypolyorganosiloxanes which suppresses the formation of cyclic organosiloxanes and reduces the amount of catalyst-deactivating agent.